1. Field
The disclosed concept relates generally to circuit interrupters, and in particular, to circuit interrupters including temperature based and current based trip mechanisms.
2. Background Information
Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Circuit breakers typically include separable contacts. The separable contacts may be operated either manually by way of an operator handle or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Fault conditions in circuit breakers are generally detected either by sensing the current flowing through the protected circuit or by sensing the temperature of conductors in the circuit breaker. Circuit breakers generally employ a mechanism such as an electronic trip unit or a magnetic trip unit to initiate a trip based on the current flowing through the protected circuit. Trips initiated by an electronic trip unit or magnetic trip unit are usually either instantaneous or initiated after a predetermined delay. A current sensor is used in the circuit breaker to sense the current flowing through the conductors in the circuit breaker. Some circuit breakers have used a current transformer to harvest power and sense current. However, the current transformer can be relatively large if it is designed to avoid becoming saturated at the rated current of the circuit breaker.
To initiate trips based on the temperature of conductors in the circuit breaker, circuit breakers have employed bi-metal based mechanisms. More particularly, current in the protected circuit flows through a piece of bi-metal in the circuit breaker causing the temperature of the bi-metal to increase. As the temperature of the bi-metal increases, the bi-metal bends. Once the bi-metal bends a sufficient amount, a mechanism is triggered, such as a latch being released, which initiates a trip.
Temperature based trips are generally caused by a sustained overcurrent condition in the protected circuit which causes the conductors in the protected circuit to increase in temperature. However, it is difficult to precisely control the amount of time and the amount of current needed to initiate a temperature based trip. It is particularly difficult to calibrate bi-metal based trip mechanisms which can vary significantly in their performance.
U.S. Patent Publication No. 2014/0078633 discloses a temperature sensor used in conjunction with an electronic trip unit to provide temperature based trips. However, placement of the temperature sensor remains a concern as placing a temperature sensor directly against a conductor can damage the temperature sensor while placing the temperature sensor proximate to the conductor can cause a time lag between the temperature sensor readings and the actual temperature of the conductor.
There is room for improvement in circuit interrupters.
There is room for improvement in temperature based trip mechanisms.